1. (Field of the Invention)
The present invention generally relates to the manufacture of semiconductor devices and, more particularly, to the method of making a hybrid semiconductor device composed of a stack of semiconductor integrated circuit boards.
2. (Description of the Prior Art)
The degree of integration of semiconductor circuit components per chip, that is, the density to which semiconductor components can be fabricated in a single chip, has rapidly increased because of improvement in micro-processing technology and design technique. As a result of these advancements and in an attempt to provide a high-density, high-performance semiconductor IC device, a three-dimensional integrated circuit is currently devised without altering existing design requirements. The three-dimensional IC device currently manufactured is available in two types. One is the monolithic type which comprises, as shown in FIG. 5 of the accompanying drawings, a single silicon substrate 10 on which insulating layers 11 and semiconductor layers 12 are alternately stacked one above the other by the use of a chemical vapor deposition technique or the like. The other is the hybrid type which, as shown in FIG. 6 of the accompanying drawings, comprises a plurality of two-dimensional chips 14 mounted one above the other, each neighboring two-dimensional chip 14 being electrically connected by a flip chip bonding technique or the like through a plurality of connecting electrodes 15 that extend in a direction along the thickness of each chip 14.
Comparing the manufacturing technology and the design, the hybrid semiconductor IC device 16 shown in FIG. 6 has fewer problems than the monolithic semiconductor IC device 13 and possesses a wider range of applications.
However, the hybrid semiconductor IC device known in the art has a detrimental problem in that, since the connecting electrodes 15 are not insulated the pitch between each neighboring connecting electrodes 15 can not be smaller than 200 .mu.m. This problem hampers a high-density electrode connection.
In order to obviate the problem discussed in connection with the hybrid semiconductor IC device shown in FIG. 6, a manufacturing process such as shown in FIG. 7 has been proposed. As shown in FIG. 7, the method begins with step (a) at which a plurality of connecting electrodes 21 are formed on an upper surface of an IC substrate 20 and a protective layer 22 is subsequently formed entirely over the upper surface of the substrate 20 so as to cover the connecting electrodes 21. At subsequent step (b), portions of the protective layer 22 are perforated in a depth sufficient to permit the respective electrodes 21 to be exposed to the outside through the resultant perforations 23, followed by the insertion of conductor pieces 24 through the perforations 23, each of the conductor pieces 24 being of a sufficient size to permit a portion thereof to be exposed outwardly from the protective layer 22. After the insertion of the conductor pieces 24, and as shown by step (c), a resinous bonding agent having an electrically insulating property is applied over the protective layer 22 to form a bonding layer 25 overlaying the conductor pieces 24, followed by step (d) at which the bonding layer 25 is etched off until respective upper regions of the conductor pieces 24 are exposed to complete a single semiconductor IC unit 26. Then, at step (e), another semiconductor IC unit 26' having an IC substrate 20', electrodes 21', a protective layer 22', conductor pieces 24' and a bonding layer 25' all formed in the manner as hereinbefore described in connection with the semiconductor unit 26 with reference to steps (a) to (d) is placed over the substrate 26 with the conductor pieces 24 in the semiconductor unit 26 exactly aligned with the conductor pieces 24' in the semiconductor unit 26' and the assembly is subsequently hot-pressed to allow the bonding layers 25 and 25' to be bonded together. After hot-pressing, the conductor pieces 24 in the semiconductor unit 26 and the conductor pieces 24' in the additional semiconductor unit 26' are electrically connected together.
According to the illustrated method of making the hybrid semiconductor IC device, to achieve satisfactory and reliable electric connections between the conductor pieces 24 in one semiconductor unit 26 and the conductor pieces 24' in the other semiconductor unit 26', it is essential that the upper region of each of the conductor pieces 24 and 24' must be flat and level with respect to the etched outer surface of the corresponding bonding layer 25, that any one of the substrates 20 and 20' must be warp free and flat, and that the semiconductor units 26 and 26' must be uniformly pressed together over the entire surface during the hot press step.
Failure to fulfill these requirements result in unsatisfactory electric connection between either some or all of the conductor pieces 24 and 24' which in turn reduces production yield and reliability. However, it is very difficult using conventional techniques and equipment to accomplish these requirements. Moreover, since the temperature and the pressure used during the hot press are fairly high, it often occurs that some or all of semiconductor elements formed on each of the substrates 20 and 20' of the respective semiconductor units 26 and 26' tend to be damaged or, at least, adversely affected.